1. Field of the Invention
The field of the invention is that of geophysical data acquisition. More precisely, the invention relates to equipment for analysing sea-beds.
In particular, the invention relates to the oil exploration industry using the seismic method, but may be applied to any field using a geophysical data acquisition network in a marine environment.
2. Background of the Invention
The geophysical data acquisition operations usually use sensor networks (called “hydrophones” for seismic data acquisition in marine environments). To collect the geophysical data in the marine environment, one or several immersed seismic sources are activated to sweep omnidirectional seismic wave trains. The wave trains generated are captured by the hydrophones previously mentioned, the latter being positioned along the length of the cables to form acoustic linear antennae that are commonly called “streamers”. Usually, the geophysical data in the marine environment is acquired using a series of streamers towed by one or more ships.
In practice, the aim is to analyse a sea bed with a minimum number of passes of the ship in the zone concerned. For this purpose, the width of the sensor network is increased as far as possible, which implies using a large number of streamers. The problem of the streamers location is therefore particularly sensitive given that their length varies between 6 and 15 kilometers. Indeed, the streamers are subjected to different external forces of variable nature and strength, such as marine currents, wind, waves, . . . .
These external constraints cause geometrical deformations of the streamers that need to be anticipated so as to navigate with greater safety and to ensure satisfactory coverage of the study zone for which the sea-bed is to be characterised. On one hand, when obstacles such as an oil rig are to be avoided, the network of streamers may have transversal drift and consequently there is a risk of collision is harmful for the safety. On the other hand, the network of streamers may have a different form from one line to another of the ship in the study zone, and these deformations are variable in time and can thus cause “holes in the coverage” that need to be filled by extra passes of the ship. These passes cause increased costs of the acquisition operations.
It can therefore be understood that there are obvious requirements for the improvement of the safety and of the reduction of the costs. However, to satisfy these requirements, the influence of the current on the deformations of the streamers needs to be characterised.
A method for simulating the positioning of a streamer and for aiding to the navigation is proposed in the prior art, described in the patent document published under the number FR-2 807 842. The purpose of this technique is to model the form of the streamers by firstly determining the current that is likely to interact with the streamers. According to the technique described, the method for simulating the positioning of a streamer comprises a phase for determining the temporal and spatial variations of the current. After this phase, the method permits current “objects” to be obtained, used to calculate the form of a streamer.